"Humanity is a virus." - almost Agent Smith Cultures are born, spread, wane and die. Virus' are the same. Cultures thereforce have some of the same properties as virus' For a culture to be successful, the number of people-groups adopting the culture must be greater than the number of people groups abandoning the culture. If a culture belongs to exactly one people group it is fragile. Successful cultures spread.
Here's where it gets meta: Successful cultures influence the people group to encourage their spread:
So there we have it. Humans may not be a virus, but perhaps humanity is.